Frequency selective network



March 3l, 1953 G. v. ELTGROTH 2,633,529

FREQUENCY SELECTIVE NETWORK Filed May 5, 1950 5 Sheets-Sheet l ngi /aa% J6 J4 MUM n 50A fao# 20077 Jaa'f n P5 FF RES/YH/YCE l I ro I 00L-000617700' 00%,35/0 1N VEN TOR. v /272 274i [j Pfl/75E Pf/,qsg

swf-'rsf T sly/Fran March 3l, 1953 G.A v. ELTGROTH 2,633,529

FREQUENCY sELEcTvE NETWORK Eiled May s, 195o 5 sheets-sheet 2 IN VEN TOR.

f -'a -M -3 +/25 +250 y WML March 31,1953 G. v. ELTGROTH r 2,633,529

l FREQQENCY SELECTIVE NETWORK 3 5 f l 543' 333 conf/m M8' J 375 My. J70 574 375 n n 365 l 1w" 376 T INVE ToR. 239710.

March 31, 1953 G. .v. ELTGROTH 2,633,529r

FREQUENCY sELEcTvE NETWORK Filed May 5, 1950 S'Sheets-Sheet 4 l INVENToR. M15 @uw G. v. ELTGROTH 2,633,529

Maid', 31, 1953 A FREQUENCY sgLECTI'vE NETWORK 5 Sheets-:Sheet 5 vFiled May S5,r 1950 INVENTOR.

:radiated dissimilar material. istances, further improvement was attained by Patented Mar. 31, 1953 UNITED sTATEs PATENT OFFICE FREQUENCY SELECTIVE NETWORK George V. Eltgroth, Philadelphia, Pa. Application May 3, 1950, Serial No. 159,686

Y 17 Claims.

This invention relates to frequency selective networks and more particularly to such a network characterized by improved rejection of un.- 4desired'signals without materially affecting the fidelity of response to the intelligence modulation on the selected signal.

Since the early days of wireless communication and the use of carrier waves as a medium for the transmission of intelligence, there has been an unremitting effort to provide for more effec- .tive utilization of the always limited spectrum space and to improve the reliability with which-intelligence may be transmitted. With the old tuned radio frequency receivers, the channel spacingor separation between the assigned carrier frequencies of transmitting stations was limited by the practically attainable selectivity of circuits operating at the frequency level of the originallytransmitted carrier frequency. This was later overcome by the use of the superheterodyne principle, which relegates the problem of adjacent channel selectivity to an intermediate frequency amplifier, usually operating at a frequency lower than the carrier frequency employed in the original transmission. Such an amplifier could attain greater selectivity in practice because it could be designed for operation at a single frequency, using arrangements commercially inexpedient in an amplier which is to be continuously tunable, and because of the fundamental simplification of the problem of attaining great selectivity at the lower intermediate frequencies usually employed.

While onegroup of engineers was overcoming the obstacles to the practical realization of great.. er selectivity, another was engaged in improving the fidelity available from components operating at audio frequencies, to make the reproduced intelligence a more faithful replica of the original. Ultimately, it became apparent that the 'improved selectivity of the frequency discriminating circuits was stripping a portion oi the .initial intelligence, that arising fromthe higher .-.frequencies, from the received carrier before demodula'tion. Hence, it became apparent that channels would have to be spaced in the frequency spectrum by an amount of the order of twice the highest` intelligence frequency to be transmitted, if bothtransmitters radiated similar material, orthe sum of the respective highest frequencies to be transmitted if both transmitters In special inthe use-of single sideband transmission, but this -represents about the present limit.

' 4There-have further been many eiorts through "the years to improve the signal to noise ratio in 'fthe Areceiver output over that existing at the receiver input. "Additional selectivity improves this ',iratio only.within certain restrictions, for the selectivity cannot beincreased` without limit, else the intelligence modulation would disappear.

The most successful noise reducing system to date are the gates and limiters. The use of other transmission systems than amplitude modulation,

such as frequency modulation and pulse or pulse code modulation also afford considerable improvement, but demand so much spectrum space that such transmitters are relegated to the higher frequency portions of the electromagnetic spectrum where the bands are less crowded, but the propagation characteristics are such as to provide relatively limited range communication.

Selective circuits constructed in accordance with the principles of the invention permit great attenuation of undesired adjacently disposed signals without corresponding impairment of the transmission of sideband energy associated with the desired signal. They further reduce the effects of selective fading and afford substantial noise reduction.

It is a primary object of the invention to provide a new and improved frequency selective net- Work.

Another object of the invention is to provide method and apparatus for attenuating undesired adjacent channel signals Without correspondingly affecting the intelligence modulation on the desired signal.

A further object of the invention is to reduce the response of signal circuits to noise impulses without substantially affecting the response of such circuit to a desired signal. Y

Still another object of the invention is to permit the closer spacing of intelligence bearing carriers within the available frequency spectrum.

Yet another object of the invention is to permit the transmission of more intelligence within a given frequency band.

Another object of the invention is to provide suitable auxiliary apparatus for periodically absorbing energy from oscillatory circuits.

Other objects and advantages of the invention will in part be disclosed and in part be obvious when the following description is read in conjunction with the drawings in which: Y

Figure l schematically illustrates a circuit from which the basic aspects of the invention. are readily understood,

Figure 2 illustrates the voltage wave which may be observed across the circuit of Figure 1,

Figure 3 illustrates graphically the envelope of the signal observed across the circuit of Figure 1 when excited by a signal having a frequency other` than the desired signal, I

Figure 4 graphically illustrates comparativ frequency responsive curves for a conventional circuit and the circuit of the invention,

Figure 5 is a vector diagram illustrating the vector relation between the carrier and the sidebands of an amplitude modulated wave,

Figure 6 schematically illustrates a receiving system incorporating the principles ofv the invention,

Figure 7 illustrates a transmitting system which may be used to double the amount of conveyd intelligence.

Figure 8 illustrates in block form a receiving system useful in conjunction with the transmitting arrangement of Figure 7,

Figure 9 schematically illustrates a controllable energy absorbing network utilizing an electron beam,

Figure 10 schematically illustrates an energy absorbing network utilizing unilateral conductors,

Figure 11 graphically illustrates the relation y.between .the observed envelope and the carrier phase position,

Figure l2 illustrates the improved' selectivity realized fromY cascading the described. selective systems,

Figure 13 illustrates in block form a receiver employing .cascaded selective .networks of' the .type described, and

Figure 14 illustrates in block form a receiver employing .cascaded selective networks of the type described in which one or. more resonant circuits within such network may be detuned from the desired signal frequency.

Considering the simple circuit of Figure l, now, it is seen to comprise in inductance 20 across which is shunted a capacitor Y2 2 paralleled by a ,switch 24. Assuming the `switch 2li to be open, the circuit is recognized as the familiar into any further circuits desired ,for the utilization thereof.

Assume that we excite the circuit 2B, .22 at its resonance frequency with the switch 213 open. There will be observed across this circuit a voltage waveform such as that observed at 32 in Figure 2. As is well known, the amplitude of the voltage appearing across the capacitor `22 is .Q times the voltage which would be observed across `the terminals of the coil 20 in the absence of such capacitor. In the ensuing discussion,

.it will be convenient to think of Q in terms of another definition, the ratio of stored energy to'energy dissipated per cycle.

VLet us now suppose `that it is` possible to operate the .switch 24 at a very high rate of speed, such that it closes twice for each oscillatory cycle of the voltage illustrated and remains `closed only momentarily. It" these closures are spaced in time by wradians andv occur at 0, 1r, 2f, 31x, etc.. that is, only when the voltage across the capacitor 22, and consequently `the enero-,v stored therein, may be expected to be zero, it will -not appear to the circuit that anything has changed, for if the switch is closed only at a voltage zero, no current flows therethrough and no energy is removedV from the circuit. If the switching frequency is now maintained constant and a signal wave of another freouency, still close enough to resonance 'to be magnied by the elect of the circuit tuning, it is obvious that "the voltage will not always beA zero at` the time the -switch is closed and, when it" is not, the stored energy in the capacitor 22 is dissipated, materially reducing the circuit magnification factor for such non-isochronous Waves. For example, it. closes at ai point on the interfering wave correspondingto an angular value of /1r/6 (sinf/S/:l/z), then V4 the stored energy is dissipated and the magnification factor is reduced to 2,. since this action takes place twice each cycle. Now, the relative phase between the .interfering wave and the switching action is subject to progressive variation, so that the attenuation suffered by such energy is subject to Acorresponding variation. When the switch closure occurs at the peak of the wave, all the stored energy is absorbed, whereas, When. the capacitor .is exposed to discharge at' the voltage zeros, no continuous attenuation iste be noted. However, the voltage across a tuned circuit' rc- ;uires an appreciable time to build up or"'decay to its nal value, so that, even though there are periods when the interfering wave is not eX- posed to supplemental energy loss, the time duration of these periods is Vsufficiently abbreviated that a substantial Vinnn'ovementl in performance may be realized.

The envelope of the increasing voltage vacross a tuned circuit in response to the sudden application of exciting energy is of the form (1 -eXp-nw/magnication). Considering that six degrees before the voltage zero is reached and six degrees after, the capacitor discharge consumes 1,610 the stored energy, it is seen that for purposes of a rough estimate it may be assumed that the time required for a relative phase shift of twelve degrees is available for voltage buildup. Taking a relative frequency difference of 1%, it is found that the peak attained by the envelope is of the order of '70% of the value it would attain Without benefit of Asuch loading, reflecting an appreciable improvement in selectivity. The general shape of such an envelope, yplotted for a frequency difference of 1% is shown at 34 in Figure 3.

It is, of course, impractical in the present stage of the art to produce capacitor discharge intervals which are infinitesimal in duration compared tothe period of the signal Wave. Brief consideration demonstrates that this does not prevent the realization of the advantages of the invention, for we are accustomed to dealing' with circuits having appreciable losses and `it is possible to avoid adding excessively to suchI losses with practical discharge times. The following table gives the proportion of energyv dissipated for the indicated angular displacement between the time of capacitor' discharge and the time: of

' voltage zero:

The frequency-.transmission characteristic of such. a circuit isshown in Figure 4, the curve^36 Arepresenting the selectivity characteristic ofi a strength than the other. vstronger sideband is without counterpart in the E conventional circuit of inductance and capacity resonating at 100 kcs., while the curve 36 represents the single signalselectivity characteristic of the same circuit with the periodic loading described herein based upon the ratio of the peak amplitude of the envelope 34 to the steady state voltage developed by a signal isochronous with 'the circuit resonant frequency and the switching frequency. It is of interest to consider Whether, as in the case of the conventional circuit, the single signal selectivity characteristic vcuit is timed to place the discharge intervals substantially at the voltage zeros, there is no distinction between the modulated and unmodulated wave. This is to say that for an amplitude modulated wave in which both sidebands are present, the response of the periodically loaded circuit to the composite signal is given by the response curve of the basic circuit of inductance and capacity without auxiliary loading for the carrier and all symmetrically disposed sidebands. That this is not true for a .signal in which one side band has been suppressed is evident from a consideration of the fmathematical expression therefor, which reveals a shifting in the position of the axis intercepts. In the case of signals asymmetrically disposed in the frequency spectrum, the combination of functions required to place the Voltage zeros always in coincidence with the zeros of the selected carrier does not exist. Essentially, therefore, the circuit described is vested with the unique .property of responding without additional attenuation to the selected carrier and to signals symmetrically disposed in the frequency spec- .trum about that carrier and having the phase relations characteristic of double side band amplitude modulation, while presenting the far more selective characteristic of 38 to signals not possessed of this characteristic.

In the case of radio reception over long distances, it may often occur that the various components of the composite signal are subjected to different attenuations, whereby one sideband arrives at the receiving location with greater To the extent that the phase is evident from Figure 5, in which the high frequency sideband is represented by 40, the .low frequency sideband by 42, and the carrier vector by 44. So long as the sideband Vectors lie in the first and fourth quadrant, the carrier '.is reinforced, and when they swing into the zsecond and third quadrants, the carrier is opposed, thereby producing the variation in carrier intensity characteristic of amplitude modulaftion. Y

Referenceis now made to Figure- 6 for the 6 'schematic diagram off'areceiving system embodying the principles of the invention.` Signal energy intercepted by the antenna 46 is coupled into the secondary circuit of a radio frequency transformer 48, which may be tuned to the desired signal frequency by a capacitor 50. The resonant circuit is completed by the bypass capacitor 52 connected between the low potential end of the transformer secondary and ground. A resistor 56 provides -avdirect current grid return to a'source of automatic gain control voltage. The signals appearing across the capacitor 50 may be impressed on the signal control grids of a pair of conventional mixer valves V54, 58,

which are also provided with space charge elec- -trodes maintained positive with respect to their associated cathodes, and oscillatorvcontrol grids connected together and to the winding of the oscillator coil 60. The cathode of the valve 54 may be returned to ground through a bias resistor shunted by a capacitor, while the cathode of the valve 50 connects with a tap on the oscillator coil in the usual manner, to generate sustained oscillations in familiar fashion.

The anode 62 of the mixer 58 may be connected with a source of positive potential through a circuit including the paralleled capacitor 66 and inductance 68. For reasons which will later appear, it is desirable that the ratio of the resonant frequency to the Q of the input tuned circuit 'be greater than the ratio of the resonant frequency to the Q of the circuit in the anode circuit of the mixer.

The anode 10 of the shunt valve 12 and the anode 14of the compensating valve 16 are also connected with the anode terminal of the tuned circuit 66, 68, and signals appearing thereacross are coupled to the control grid of the amplifier valve 82 by capacitor 18. The valve 12 is caused to optionally appear as a controllable resistance with a comparatively low value by coupling the controlV grid 84 through capacitor 86 with the anode 10. A resistor 88, chosen of such magnitude as not to introduce appreciable phase shift and to load the tuned circuit 66, 68 negligibly, connects between the control: grid 84 and a suitable source of bias potential. The space charge grid'90 of the valve 12 and92 of the valve 16 may be returned to a suitable source of exciting potential, chosen for illustrative purposes to be volts. The control grid 94 of each of the valves 12, 16 is provided respectively with an auxiliary control or suppressor grid 98, |00, connected by the lines 99, |6| with a source of switching control-potentials to be later described. The voltages developed across the tuned circuit 66, 68 are coupled to the control grid 80 of the 'conventional amplifier valve 32, which, through the intermediate frequency transformer, drives device, such as the loudspeaker |06.

The switching control potentialsv are basically derived from the operation of a multivibrator |08, including the valves ||0 and v| I6, illustrated here as triodes.

The anode ||2 of the valve ||0 is coupled through the compensated load H4 to the supply bus |25, maintained as a positive K potential of 125 volts with respect to ground, the anode l 8'of the valve ||6 being similarly coupled through the compensated lload |20. The magnitude of the series ,peaking nductancein the 7 compensated loads may be determinedin acfcordance; with the4 usual rules. governing. the designv of.l extendedrange ampliers..

The` anode ||8 ofV the, valve l Himay.v be.- coupled throughv the capacitor |22- to. the control. grid |26 of thev valve. H0, the grid return being pro.- vided-I by the. connection. of a resistor |34. to-the movable. tap on. the potentiometer |38 bridged between ground andv plus-.125 volts. The tap of the potentiometer |38 mayy be grounded for high frequency currentsy by capacitor. |40. In like manner, the anode |.|2 of. the. valve H8 may be coupled through the capacitor |24 with the control grid |28 ofthe valve IIS,` whose grid. re.- turn is provided by the resistor |36 connected with the..movable. tap on. the potentiometer: |42, bypassedto ground. through capacitor |44. The cathodes |30 andr |32 of the valves ||.0 and H6 arer connectedV with. ground through resistor-- |50 paralleled by capacitor |52. This circuitV may have arelatively longtimeconstant, such. as. 0.1 second to smooth out controlaction on themulti.- vibrator frequency.

A diode |54 has its cathode connected with the control grid |26, and another diode. |56 has its cathode connected with the control grid |26, the anodes of saidv diodes being connected together and to-one endof` the resistor |46, having its other endconnected with the movable tap of potentiometer |43y bridged between ground and minus 50 volts. In addition, the tap of the potentiometer |48 may be connected with ground through a bypass capacitor |58.

Signals passing. through the diodes |54 and |56 are coupled through the capacitor |60 with a single shot multivibrator or delay flop |68, acting. as a pulse former, to develop impulses with a repetition rate determined` by the multivibrator |08 and a duration. determined by the constants of. the delay flop circuits. For the purposes ofv illustration, it may be assumed that the operating frequency of the multivibrator is 190 lrcs., and thatthe delay op delivers pulses with a length of the order of 0.2 microseconds.

The delay flop |68v may include a valve |12 having an anode |14 connected with the supply bus |25 through the compensated load |16 and coupled by a capacitor |84 with the control grid |82 of a companion valve |64. The anode |62 of the.. valve |64 is also coupled with the supply bus |25 through a compensated load. |66, and is connected with control. grid |16 of the valve |12 through capacitor |90 and resistor |88, said resistor beingl shunted byfaspeedrup.4 capacitor |86. The. cathodes |.18..and. |86 of the valves |64. and |12; connected with ground.. and. the grid |62 is connected. with a source oi"Y negative; bias potential through resistor |62. The magnitude of the bias normally supplied to the control grid |82l is such` as to maintain this valve normally nonconductive. On the other hand, the control grid |16 of the valve |12 is connected through resistor |64 with the movable tap of a potentiometer |96 bridged between the supply bus |25 and ground, so that the valve |12 is normally maintained in av conductive state.

The `anode |62 of the valve |64 is further connected. with thelcontrolgrid 202 of the coupling valve 204 by a capacitor 200, the grid end of the capacitor being. returned to ground througha resistor 2 8. The anode 208. of the coupling valve 2 04 may be. connected. with a source of: anode voltage through they compensated load. 2 |0,vwhile its, cathode 26.6 may be connected. with ground .through a compensated load 214. The

control line, 99 is energized from. the, anode 208 of the coupling valve 204 through the. coupling capacitor` 212, and. is. connected through the resistor 2f|1 with themovable tap of the potentiomfeter 220, grounded for. high frequency currents by capacitor 222. Similarly, controlV line UJI. is energized from` the cathode 206 of the coupling Valve 204 through the coupling capacitor 2|6, and. connected with the movable tap of the po.- tentiometer 226 through resistor 224, whose tap end is further linked with. ground through. the bypass capacitor 228.

Signals appearing at the. anodel 268 of the coupling valve 204. are.. coupled through capacitor 230 to the auxiliary control grid 252 of the con.- trol gate valve. 234, the. grid return for this. grid being provided by. resistor 268 connected. tov a source of potential normally maintaining. this valvev cut off. The space. charge grid 244 of. the valve' 234 may be excited from a plus 125 volt bus, while the. anode 23.8 may be connected with an anode source through the load resistor 240 compensated or uncompensated. Reference signals may be applied to the control grid 236 of the valve 234 fromA the phase shifter 262 through the capacitor 264. A nominal negative bias potential may be applied to the control grid 236 through the grid resistor 2.66. The reference signals may be obtained by connecting a highly selective intermediate frequency amplier 260 with the anode 64 of the mixer valve 54, and applying the output of this amplifier to the phase shifter. 262. It is permissible to use any of the Well known methods of enhancing the selectivity of the amplifier 260, for it is not required that this amplier pass intelligence controlled frequencies, but merely energy at the carrier frequency for reference purposes.

The anode 238 of the valve 234 is further coupled with the control grid 252 of a control valve 254 by a coupling capacitor 246, a diode 248 being also connected between grid 252 and ground, with its cathodeconnect'ed with the control grid 252 and its anode connected with ground. A resistor connected in parallel therewith provides a direct current return path. The anode 256 of the control valve 254 may be connected with the supply bus |25, and its cathode 258 is connected with the ungrounded end of resistor |56.

It is to be understood, in this and the remaining figures, that the usual heaters may be associated with thermionic cathodes, as required, such heaters and energizing circuits therefore having been omitted from the illustrations in the interestof clarity, their general nature being Well known. The required direct current operating' potentials for exciting` the balance of the elements of the valves maybe derived from any suitable power supply connected with a voltage dividing network, such as that illustrated at 260.

The circuit of' Figure 6 functions in relatively straightforward manner, the signals from the antenna 46 being applied to the transformerll, which is tuned to resonance with the desired signal by adjustment oi the capacitor 50. The mixer valve 58 generates sustained oscillations in the oscillator coil 60, whose frequency may be adjusted by variation of the adjustable capacitor 6| connected with the said coil 60. The tuning capacitors 50 and 6| may loe-mechanically coupled for simultaneous variation and so designed that the difference between the selected signal frequency andthe frequency of. the locally generated oscillations is constant, and equal tothe selectedintermediate frequency, as is. customary pedients for enhancing its frequency discriminating` properties as required and generally indicated by the balance of performance and ecce nomic factors. This high degree of selectivity is permissible, as the control channel including the amplifier 260 is not required to handle intelligence frequencies, but need pass only the carrier itself, for synchronizing purposes.V From the outp ut of the amplifier 260, the energy of the selected frequency is fed into the phase shifter 262, of which there are many types available, delivering itto the synchronizing circuits whose functions will bev later described. The phase shifter 262 is adjusted for minimum loss of the desired signal in the signal channel. @The tuned signal circuit 66, 68 in the anode circuit of the mixer valve 58 is excited by the beat frequency energy to which it is tuned. For the purpose of the initial explanation, it may be assumed that the selected beat and the free oscillation frequency of the circuit 66, 68 are isochronous. The amplified voltage appearing thereacross lis further amplified by the valve 82 and demodulated by the diode 81. Demodulation and Work circuits suitable for use with modulation frequencies in the audio frequency range have been shown in the illustrated embodiment of the invention, it being obviousthat these new principles may be applied with advantage in configurations characteristic of any other range of modulation frequencies. The demodulated signals across the potentiometer |02 are applied to an amplifier |04 and load device represented by the speaker |06. 1

`In addition to these conventional signal circuits, the shunt valve 1| and compensating valve 16 are also connected across the circuit 66, 68 in the manner already described. The valve 12 is normally maintained in substantially inoperative condition, while the bias potentials applied to the valve 16 are such that it normally passes anode current.. However, the valve 16 is of a multiple electrode type, such as a 6AS6, presenting-;a very high A. C. anode impedance, so that it loads the circuit 66, 68 only negligibly, despite its normally operative condition. Control signals applied to the valves 12, 16 periodically change the impedance across the circuit 66, 68. Y These control signals derive initially from the multivibrator |08, of conventional structure. "Ihe free running frequency of thevmultivibrator |08 and time division between the two valves ||0, ||6 thereof, are controlled by adjustment of the tap position on potentiometers |38 Vand |42. Moving either tap toward the more positive potentiometer terminal increases the free running frequency by decreasing the time during which the associated valve remains in the nonconductive state. These potentiometers are preferably adjusted so that the transitions of state between the" two multivibratorvalves occur 1r radians apart, referred to the signal wave to be selected, although the advantages of the inven tion may also be enjoyed if the transitions occur more'than 1r radians apart, so long as the spacing of the transitions approximates an integral multiple of 1r. Such greater spacing, however, diminishes the degree of improvement in se# lectivity which may be attained. Separation by lessthan 1r radians again reduces the selectivity improvement. Variation in the current flowing` through resistor |50 changes the free running frequency without affecting the time division.l This effect is used to control the multivibrator |08 from the control signal channel. A Each time the .anode ||8 of the valve ||6 suffers a negative excursion, the signal transfl ferred through the capacitor |22 drives theco'ntrol grid |26 of the valve ||0 to cut-off" and initiates conduction through the couplingA diode |54 which transmits the signal to the diode end of the resistor |46. Adjustment of the .tap on potentiometer |48 determines the amount o f negative excursion required to initiate signal transfer, through its fixing of a threshold volt.- age required for. conduction. The diode |56 transfers signals from the control grid 5|'28 to' the diode end of resistor |46 in like fashionl The'use of the diodes |54, |56 grows out of anumber of considerations'. Primary` among these is the fact that the transition time of the negative anode excursion of the multivibrator valves is much less than the .transition .time involved in the positive excursion. Itis, there# fore, desirable to use the negative stroke. For best results, it is desirable that the transfer .of operation between the shunt valve 12 andthe compensating valve 16 occur every 1r radians at the signal frequency.. Userof the negative stroke from a single -side of the multivibrator would require, if this condition were to be satisfied, that the multivibrator operate at twice the signal frequency. As the frequencies'which wouldbe involved in this instance are attainable in a multivibrator only with the exercise of con,l siderable care and skill in design and fabrica'- tion, it is convenient to simplify the problem by interposing the diodes referred to, whereby two negative strokes per operating cycle are available. the need for isolating the two circuits, and the adjustable threshold afforded bythe potentiometer |48 eliminates the effects of minor trans,- ients. Y y

The capacitor |60 is small enough to perform a differentiating action onthe signal vdelivered by the diodes |54,. |56, whichisapplied to the anode |62 of the normally nonconducting .valve |64 in the pulse former |68. Thenegative impulse triggersl a process initiating the transfer of conduction from the valve |12 to the valve |64,` where it remains until discharge of the capacitor |96. The time required for this vdischarge, and Vconsequently the length of the triggered impulse, may be regulated by adjust- `mentk of the potentiometer |96. `When the .capacitor has discharged sufiiciently',con`l duction through the valve |12 is resumed,4 while that through the valve |64 is cut olffY As already mentioned, thelength of this interval'may be about 0.2 microsecond, when-operating Witha signal channelv intermediate frequency Vof kcs. This representsan angle of somewhat over seven degrees atV the signal channel frequency. Reference has already been-made to the tendency ofthe positive stroke vofthe anode excursionin a multivibrator to have a less steep risethan The use of two diodes is required by il the negative stroke. The reason for this is 'twofold. First is the fact that, during the negative stroke, the valve anode resistance is in parallel with the load impedance, providing minimum tim'e 1constant and vthereby improving the speed of lcircuit response. vSecond is the consideration that, during the positive stroke, the grid of the valve which is being driven into the conductive state becomes positive and also conducts, whereby 'the grid 'coupling capacitance is leifectively yconnected across the anode circuit of vthe valve being driven into the 4nonconductive state. This slows up the rise of the anode voltage to its rest value. :Capacitor |86 and resistor |88 are interposed in Athe 'coupling link between the anode |62 of the valve |64 and the grid |16 of *the valve 112 to minimize this 4eifectl The resistor '|88 is large `with respect to "the conductive impedance 'of the 'control grid |16 when it is positive, and the capacitor "|86 is lquite small, passing only `the initial surge occurring at the vanode |62. It is comparable with the control grid input capacitance. This permits the flow of grid current which would otherwise ef- Iectively connect the ncapacitor |93 between the anode |62 and ground, thereby speeding up the positive-going voltage rise.

The negative surgeappearing at the anode |62 is impressed -on'the control ygrid 282 vof the lcoupllng valve '204, to produce a negative-going impulse of rcorresponding "duration 'on the line ||J| andsI positive-going impulse 'fof like duration on vtl'ie-lineQB. Thepositive `impulse on line-99 permits conduction through the 4shunt valve 12, while 1'the Inegative-going impulse 'on line rinterrupts lconduction through the compensating valve 16. The `potentiometers 220 and 226 are so adjustedthat, yin the absence of signal across the circuit"66, '68, the current drawn 'by the valve 12 `during conduction is Vessentially -equal to that normally drawn bythe valve 16, whereby the inec'tion of new voltages vinto the signal circuit y(it, 68 'is avoided, since a constant current flows therethrough, only thepath varying between the shunt valve 12 and compensating -valve 16. If the switching impulses on the lines 99 and are properly synchronized with the desired signal stimuli, lthe shunt valve will Vbe active only when the voltage 'across the elements 66, t8 is "passing through zero, and lthe circuit reacts in the manner of a normal inductance-capacitance combination. If the voltage at anode 62 is positiveofits'reference value, this is coupled through capacitor 86 to the control grid 84 of the shunt valve "12 and increases the current drawn by this Avalve during its active period. Conversely, if the `voltage at anode 62 is negative of its reference value,'this is coupledthrough capacitor-86 to the control grid 84 to decrease the anode current. This is the equivalentof connecting a resistor of value I/Gm across the combination 66, 68 during Ythe positive Yexcursion of the line 99.

A".llhe presence of vthe compensating valve "I6 maintains the continuity of current now through thecircuitt, 68 and avoids introducing disturbing signalsinto the signal channel. Since, inthe example described, the switching action occurs twice per signal cycle, such a disturbance would have a y'frequency double the signal frequency, and might be eliminated by the action of succeeding tuned circuits. However, even in this Vcventythe task of eliminating the disturbance is simplified by `minimizing its initial magnitude through the yuse of the compensatng valve. Greater spacings of the switching action 'make l2 the use of a compensating valve even more desirable.

Because the frequency-transmission characteristie of the circuit is dependent -upon the phase relation between the control waveforms appearing in the output of the coupling valve 202| and the selected signal energy,` either highly .stable transmitting and local oscillators must :be employed, or the transmitted frequency must be used rto govern 4the operation of lthe source of control potentials. `The latter :expedient has been applied in the illustrative embodiment.

The valve 234 is employed as :a .control 4xgate, with its control grid 236 excited from the output of the phase shifter 262 in the control :signal channel. Each time the auxiliary lgrid P232 is driven positive during the-actuation of the shunt valve 12, an anode current pulse passed by-the valve 234, whose magnitude is controlled Eby ithe phase relation between vthe Acontrol channel signal `and Vthe output of Athe coupling valve 2270i. The corresponding negative-going anode voltage excursion is coupled through capacitor 264 to the diode Zilliwhich passes it to groundfcharging the capacitor 264, whereby the average potential applied to the grid v242 of the -va'lve :254 ismade positive an amount depending iupon the peak negative excursion. An -increase in :the 'peak negative voltage excursion increases the flow of current through vthe resistor V| 5|), tending to cause the frequency of the `multivibrator to `decrease and re-establish the vproper frequency 4and phase relationship to the control signal and conversely. The synchronizing and lcontrol action is related to the positive slope portion of the control 4'signal delivered from the 'phase vshifter 262. Hencethe control action of the system illustrated in detail is limited to arrangements in which the periodic -shunting action occurs at a `frequency which :is equal to, or an integral submultiple of, =the lsignal frequency to be accepted. Obvious circuit modiiications extend the utility :of the control =ar rangement to other relationships between the periodic shunt frequency and signal frequency to be accepted, as for example, the excitation of the grid 232 of the gating valve 234 from one of the anodes or grids in the multivibrator |118.

Energy at the desired incoming frequency -produces a, periodic wave across the circuit 66, i6-8 whose voltage zeros coincide substantially with the time intervals during which the shunt valve I2 is actuated, and so pass on in the normal manner. Energy at other frequencies produces across lthe circuit 66, 68 a voltage iwave `which will not, in general, have a zero value at the time the shunt valve 12 is actuated, whereby a substantial portion of the energy stored in the oscillating circuit is dissipated, tc 'reduce lthe circuit response to such undesired disturbances. The net result is a considerable improvement vin discrimination against signals whose lfrequency components are asymmetrically disposed about the acceptance frequency of the system, while signals having a symmetrical composition, as produced in the amplitude modulation of a carr-ier wave, pass without attenuation beyond that inherent in the elements 66, 68.

In the foregoing discussion, it was assumed that the control signal frequency and the frequency of 'free oscillation of the combination 66, '68 were the same. Consider now the response of the system to a transient impulse, such as a step function. If `this step function arrives ata voltage zero, the resultant transient dueto ringing of the circuit 6B,-6Bis in phase with 'the de 13 sired signal and suffers no attenuation beyond that experienced from the circuit losses. If the circuit Q be 100, the transient will have died away to l/e of its initial value in about 32 cycles. If

the step function arrives at a voltage peak, the

transient is completely dissipated at the time. of the next shunting connection. Thus, an improvement in signal-to-noise ratio may be expected in the case Where the switching signal and free oscillation frequencies are the same. If the free oscillation frequency and the signal are different, say, by T16 of one percent, then the step function arriving at the signal zero suffers additional loss at each switching connection. With a signal circuit Q of 100, the amplitude of the transient will be reduced to l/e of initial value in about 16 cycles. Transients caused by step functions arriving at continue to be destroyed during the firstshunting cycle. The result is a further-improvement in signal-to-noise ratio. v Further consideration of the properties of the. circui-t described suggests interestingy possibilities for the transmission of more than one'independent train of information over a single carrier frequency without expansion of the band requirements.- Energ-y at the acceptance frequency may be transmitted or dissipated, according to its phase with respect to the periods of activation of the shunt valve 12. If the voltage peaks which would be produced are positioned in time at the active shunting intervals, the build-up of stored energy by which selective circuitsenhance or' magnify the desired signal is prevented, and the voltage of such signal will not be significantly more than l/Q of the voltage level attained in the absence of such action. On the other hand, simultaneously present energy at the same frequency, 'so phased that the zeros of the voltage wave produced thereby fall at the active shunting intervals, experiences no such attenuation and builds up to the usual enhanced level without interference. Two such carriers at the acceptance frequency may be independently modulated with dierent program material, and one or the other selected at the receiving location without material interference from the other. If tuned circuits having a Q of 100 are employed, the'relative discrimination between the two signals, lsent over the same transmission channel and occupying the same band spectrum, can'well be of the order of 40 db. Higher Q circuits provide correspondingly greater discriminating ability.

A transmitting system radiating a signal of the required type for the transmission of different program material on the same carrier frequency vwithin the same frequency spectrum is illustrated in block form in` Figure 7. the details of the block elements being sufficiently familiar to those skilled in the art to obviate the necessity for c'etailedV description of their internal configuration A common radio frequency source 210 maybe employed, a part of its output being impressed on the'v ampliiier'valve 218 through the phase shifter 214. Another portion of the output of the source 210 is applied lto the valve 211 through the phase shifter 212. The anode circuitof the valve 211 includes the tuned circuit 28| and the secondary of the Ymodulation transformer 215 supplied with program material from a first source. The tuning of the. circuit. 28| 1S 'iii adjusted in the conventional manner, and the signal potentials applied to the transformer 215 amplitude modulate the output of the valve 211V in well known fashion.

Similarly, the anode circuit of the valve 218v includes the tuned output circuit 280 and the secondary of the modulation transformer 21B excited by signals from a second program source,

wherebyA an -amplitude modulated carrier wave appears in the circuit 280. In each case, the secondary of the modulation transformer is shunted by a suitable radio frequency bypass capacitor.

The signals in the circuits 28| and 280 are cou-l pled to and radiated from the common antenna 219.

The only critical requirement in this transmitting system is that the respective carrier func-- tions associated with the individual programs are orthogonally related, which is to say that the carriers are substantially ninety degrees out of' phase with each other. In the illustration of Figure '7, this may be attained by proper adjustment of the phase Shifters 212 and 214,- for eX- ample, exciting the valve 211 45 leading and they valve 218 45 lagging. The same result may bel chronously controlled radio frequency sourcesl might be used. These details are not material.l so long as two carrier disposed in quadrature relationships areemployed.

lRadiation of the two signals from a common radiator is desirable if the effects of varying path lengths are to be avoided. Independent radiators are acceptable, however, where the spacing is heid within limits meeting the operational re'- quirements, or Where the signal separation is intended to be available along one or more selected paths.

The signals radiated from .the system just described may be separately utilized in the receiver of `Figure 8. This receiver consists of tWo systems generally similar to that of Figure r6, modified by the insertion of an additional tuned radio frequency selector stage ahead of that in the signal input circuit of the mixer. This modification is not essential but represents a conventional method of improving the image frequency rejection. The output of the tuned radio frequency amplifier 30! is impressed on the mixer 302 in the rst signal channel, this and the balance of the mixers being supplied with local oscillations from the local oscillatorv 303 providing a periodic wave having a frequency separated from the incoming signal by the desired intermediate frequency.

The output of the mixer 302 at the desired intermediate frequency is impressed on a resonant circuit, having its maximum response near the said Vintermediate frequency, incorporated in one of the block elements, and delivered therefrom to the intermediate frequency amplifier 304, which amplifies the signal and impresses it on the demodulator 306, whose output excites the vmodulation frequency amplifier 308 driving the load device'3l0. The shunt 3I2 and (if it is used) compensator 3I4 operate on the resonant circuit inthe manner described above.

l, .The second signal channel includes the mixer 305 receiving stimuli from the output of the Atuned radio frequency amplifier 30| and local I5 oscillator 303, having its output connectedqwith controL electrode 341,. focusing; electrodeand secondi anode.y 3M!l mounted. in.. one end. ofi the evacuated envelope. 3.42. When the.. proper. potentialsiare;` applied .to these-electrodes, .they pro.- ject an electron. beam pastthe. deflect-,ing plate pair 3.43, 344 to the; end of.. the envelope housing the spaced targetelectrodes 34334I.. The electrodes 3.4 |f and. 343-.` are .connected with the. other sideot the circuit 66, 68.- The. target. electrodes 3.401 34 |y arey sospaced that,.inl the absence ofp tential. across the-deflecting electrodes.343., 344, the. electronbeam 3.45.. falls. equally on bothtarlget electrodes orfalls abetweenthem, ,substantially filling the spacefbetween` the. target'. electrodes. The second. anode` 349 may be` connected with ground,.while.the focusing electrode. 3.48 iscon.- nected. withtan. adj ustable. tap on. the voltage di.- vider 3.50r bridged between ground and a. source othighlynegatiyefpotential.. Another tapof said voltage-.divider 350. is..connected.withthecathode i146, while.: the beam:A intensityelectrodel] is linked. throughv resistor. 35 I. to the. negative end ot the voltage` divider 35.0... Theicathodetap is adjusted to.impress.- on. the .-intensityf control-.electrod 341. a;- potential. normally preventing.. the emission. of electrons1 fromthe cathode 346. to develop` the beam 345;

Aecapacitor.. 352 connects-the. intensity control electrode3-41: with a-sourceof positive-going im.- pulsessuch asthe.v coupler. Valve. 234 in .Figure 6. Upon-thearrival of. a; positive-going impulse at the intensity control' electrode..3.41, the=beam 345 isffestablished. It. the voltage acrossthec'rcuit 66; 68.A is. zeroat-.thisinstant the .beam f either. fails to.touch. either. of. the -two target'. electrodes, or mpinges on.` them; equally, whereby no load. is presentedto. the. circuit.66', 66 and no. transient voltages. are. induced.. therein. It,` however,. the target e'lect1-ode` 34| and.. delecting electrode. 3.43 are" relatively positive. the electron: beamis: deected tot impingeitofagreater degree-on theJtar.. get electrodeN 34|-, effectivelydrawingcurrent from; the: corresponding terminal. of. the-y circuit 66;,68; and loadingitan amountI dependentupon the beam currentandbeam'.deection; A .relatively -negative potential.V on L the. target.. electrode 34|. and. .deflectingi electrode 3.43. gives. rise. to ,the reverse effect, corresponding, Ito.` ther currentV iiow thatwould-.take placei.through .a loadingrresistor the?.I presence; of.. such: reversed polarity.

Ifilthee desired-.balancear beam current inter.- .ception-by/ the' target .electrodes 3.43,- ,.341.' doesfnot normallyf exist in'. theistructure of. the:y vallve. of Figure:9`,-.the `beam may bevdeectedzto. asuitable extent;v using: an.. external magnetic. field, or superimposing-, the necessary deecting.. potentials on: the: deiiecting electrodes 343,.- 344-.- in. circuits of; conventional form.. When beamr current` is properly balanced betweenzthe v tar-getv electrodes, this'. arramgementds. free-1 of; the. requirement for afcompensaztingY valve and permits some simplicationofithe foregoing configurations.

Another circuitpossessed of theseadvantages isf revealed. 1n..Figure-10. In thisf arrangement,

the coil. 6.8. is'. centertapped and. connected with.

ground,Vv and: the-adjustable. tuning; capaciton 66 connected. thereacross. in. the. usual.. manner. A tiret-pair' of.' diodes. 36D 3611 is'I connectediin. series opposition across? theY leads. to: capacitor; 66.. with their: anodesw connectedtogether, and. a; secondv withithefcathodefend of-thefresistor 36.4 situatedV between. thev cathode 3'65,.of. the valvel 366, and. a supply conductor maintained at a negative potentialof approximately lvol'ts. The valve 366Lis further providedwitha controlI gri`d`361 returned to ground through. the grid. leak resistor.. 368. The. ungrounded'. end. of. the, resistor 36.8.1.1nay. be connectedthrough. afcapacitor 369 withA asource. of4 positiveegoing. impulses, such as-the pulse/former. or. coupler. valve 204 of, Figure 6 The` anode. 31101. of' thevalve. 366y is connected with a. source ot anode. potential. through. the compensatedload. 31|` and tol the control'. grid 312y ot theA valve 3131- through. thevv capacitor 3142 Theanode 3-.15ofl the valve313'may beconnected directlyl tor a. sourceV of!` anode potential, Whiler its cathodev3l16 isreturnedfthrough thecathode resistor 311 to asupply conductor. having. a neg-ative potential of approximately ten volts. The cathode end of the resistor 311 is connected'with the junction of the diodes 362, 363,*and, through resistor 318 with the control grid 312 of the valve 3.13.

Neglectng, for the'moment; the-signal voltages whichmay'appear'finthe circuit 66, 68', itis ap= parent that theva'lve 366 is normally noncon'- ductive, While the valvey 313 is normally conductive. The anodes ofthe diodes 360; 361i are therefore`A negative with respect to their' associated cathodes and' the cath'odes ofv the diodes 362, 3.634 are positive with respect'to theirassociated anodes; whereby neither-of' the vdiode combinations loads` the' circuits 36 68` appreciably,

since these potentials renderA them noncon ductive: Withthe arrival-'of'7 a' positive-going'm'- ,y pulse atthe input'ofithe valve 3662 conduction i's initiated which" makesfthe cathode end of the resistor 3'64 positive-with respect to ground and conditions the diodes 366'; 36`| forl conduction. At the same time', the current'surge-owingin the anode circuitproduces across the com'- pensat'ed load 31|" a' negative-going impulse transmitted through the capacitor314to the con'- trol: grid 3121 of the lvalve 313; Who'seanode --current is;cutv off; permitting the cathode end' of the resistor- 3.11 tolbecom'e negative and condi'- tion the' diodes 362,363' for conduction; It' the voltage across'thecapacitor 66de' passing-through a' zero` at this instant, the' operation ofthev circuit isi undisturbed. If,r however, the voltage across the capacitor 66 differs appreciablyfrom zero, anun'balance'current'willfiow through the=diode, producing a; loading equivalent 'to the connection of twice ther` forwardz resistance of the' diodes across' the capacitor-"66 duringA the interval* that the vallve' 366" is maintained: conductive and' the valve' 3.13 isnonconductive.- Two pairs ofdiodes have'. been employed' tov permit' loading the cir-- cuit during; both thepo'sitive and' negative-cycles. If it'is desired to` dispense with loading in' oneof these' senses; it is permissible to'employv a single set of diodes:

The output from the circuits 66, 68 is applied y in. the conventional manner-to a pairA of" pushpull connected valves-31 8, 31 31Which1may-'b'ethen connected With any further apparatus desired. In this arrangement, of course, it isre'quiredl that the signal potentials be lowiby comparison with theswitching voltages deliveredbythe val'ves 36 and 313; i

While theselectivity curve 38 of Figurer! represents amateria'l"improvement'over the conventional' responsey characteristic illustrated at.' 36 in that.gureithas.been found'that the unusual properties of' this. method'. off enhancing. circuit 1 selectivity also. permit` unusualY improvements. in

the selectivity attainable. This is achieved through the use of groups of such circuits. In the following it is to be particularly borne in mind that the selectivity characteristic 38 illustrated in Figure 4 is given by taking the ratio of the peaks of the output as represented at 34 in Figure 3 to the steady state output at the desired signal frequency. Broadly speaking, the selectivity curve 38 may be regarded as the poorest member of a family of selectivity curves, each existing during a particular instant in time. This is because the selectivity ratio changes continually during the operating cycle, being very high at the time when the envelope 34 is at its trough or minimum value and being relatively lower when the envelope 34 has attained its peak value. When the incoming signal differs in frequency from times the frequency with which the shunt circuit is actuated (where n is any integer), progressive slippage in relative phase position is observed, Which gives rise to the output phase envelope shown at 34 in Figure 11, with maximum attenuation occurring when the shunting periods coincide with a voltage peak across the capacitor and minimum attenuation occurring somewhat after coincidence of the voltage zero with the activation of the shunting circuit. A change in the phase of the undesired signal effects a displacement of the troughs expressed on an absolute time scale. The cascading of selective circuits of the type described herein utilizes this effect to obtain an enhanced selectivity characteristic which is not a product of the selectivity characteristics of the cascaded circuits, in the mathematical sense, but which is, rather, the unique result of the time-staggering of the selectivity characteristics.

This will be best understood after consideration of Figure il in which the output envelope 34 appears on a reference grid. It will be recollected that this is the output which is derived from a circuit of the type described when the impressed frequency and the frequency of actuation of the shunting circuits are non-isochronous. The characteristic 400 in Figure 11 illustrates the time variation of the modulation of the envelope amplitude if a wave -of the same frequency but displaced in phase by ninety degrees is impressed on a circuit of the type described in which the time of the shunt actuation is the same as that producing wave of 34. If it now be required that the output of 34 pass through the circuit giving rise to the envelope 400, the envelope of the resultant wave is of the nature illustrated at 402, that is to say, the peak amplitude of the envelope has been greatly reduced.

It is possible, in a manner to be later described, to require incoming signals to run this double gauntlet giving rise to the new selectivity characteristic 404 illustrated in Figure 12, representing, not the square of the characteristic 38, as in the case of conventional circuits, but

a new function based on the foregoing phenomena.

A typical arrangement for attaining the benefits following on the time-staggering of the transmission minima is illustrated in block form in Figure 13, the block notation being employed, as the nature and interconnection of the block elements is apparent when the figure is considered inconnection withv Figures 6, 9, 'and 10. The antenna 406 is connected through the tuned-radio frequency amplifier 408 to the mixer stage 410 and, through radio frequency amplifier 409, with the mixer 411 in the control signal channel. An oscillator 405 also excites the mixer 410,1and through the phase shifter 401, the mixer 411. The output of the mixer 410 is impressed on the resonant circuit 412, having a reasonably high Q, across which there is connected a periodic shunt 414. In circuits where, as here, more than one tuned circuit is present in the signal channel before the signal is delivered to the periodically shunted resonant circuit, it is desirable that the sum of the decrements of the preceding tuned circuits not be greater than the decrement of the periodically shunted tuned circuit. In the present instance, this requires that the sum of the decrement of the tuned circuit in the radio frequency amplifier 408 and ofthe decrement of the tuned circuit in the input to the mixer stage 504 be not greater than the decrement of the circuit 506. The signals appearing in the circuit 412 are impressed on the amplifier valve 416 in whose output there is situ'- ated a further resonant circuit 418 operatively associated with periodic shunt 420. The output of the circuit 413 is delivered to detector 422, where the signals are dernodulated and the modulation impressed on the modulator frequency amplifier 424 feeding the load device 426.

The stimuli controlling the action of the periodic shunts 414, 420 are derived from a multivibrator 428 having its output connected with a pulse former 430. A portion of the pulse former output drives a coupling deviceA 432 actuating the periodic shunts 414, 420, and another portion of the output of the pulse former 430 is impressed on the comparator 434, which is also excited by the highly selective intermediate frequency amplier 436 connected to the output of the control signal channel mixer 411. The stimuli resulting from the comparison of the signals in the pulse former 430 and the signals from the intermediate frequency amplier 436 are impressed on the control unit 433 to govern the frequency and/or phase of the operation of the multivibrator 428.

The circuits 412 and 418 are tuned topresent essentially pure resistive impedances to the desired signal. The radio frequency mixer'` 410 cooperates with the oscillator 405, to deliver an intermediate frequency stimulus of the desired frequency to the circuit 412. The phase shifter 40'1 is adjustedso that the time of actuation lof the shunt circuit 414 occurs substantially at the voltage zeros occurring across the circuit 412 when a signal of the desired frequency and phase is impressed thereon. As the circuit`418 in the output circuit of the amplier 410 is similarly tuned to present an essentially pure resistive impedance at the desired signal frequency, the position of the voltage zeros in time is the same throughout the circuits 1112, 418, despite the phase reversal of the signal Waves themselves. Hence, the periodic shunt 420, driven from the coupling device 432, is similarly actuated during the voltage zeros of the desired signal wave when the phase shifter 401 has been properly adjusted with respect to the operation of the circuit 412 of periodic shunt 414.

The highly selective transmission characteristic of the combination of Figure 13 is based upon the cumulative phase shift observed in the transmission of a signal through a train of circuits theretoy varies quite: rapidlyv with. changev in., the

impressed frequency; such a. phase curve-.isf illus:- trated: on. page. L22. of. Reference Data. for? Radio Engineers, third edition,I published; by1 Eedelal Telephone and Radio Corporation. curve illustrates` that. the. phase. shift.. in a single .cr-

cuit whenV the. difference. between. the impressed.

and resonant frequencies is; off the order' of: 9.1 per cent-.is elevenv degrees.. Thus., when the-.circuit; Aglz .is passing; a signal .l per; cent; remoyed from the desired signal; frequency without at..- tenuatiomit will be. suffering: attenuation of at least one-third in the circuit 4I8, and. Grin:- verselr. At; no time. thereiore,Y will` the, response amplitude. to a signal remored by one-tenth. of chiel per cent of the.l desired signalv frequency' ex.- ceed approximately. thirty..V per cent oi the.. re.- Snchse to.y the; desi-red` signal when. circuits: hauins a. Q` ih. the range oil lill). are employed; Qonsultation of. selectiyitu characteristic listed. in the foregoing: and'. other. publications shows: that airecuericy-deviaticn of; this magnitude..- in. a, pair of cascaded circuits hal/.m22 aix-Q Qf: 10.0; will hardly produce measurable attenuation.- with, respect.. to desired, signal.. The attehuationsi. of; course. increase! quite.: rapidlyl with. further: deyiation filcih. the desired signal: tredueney because; or the stcephess f; the plane; shift..irefluency. characteristic and.. the accumulation or; summing' up ci,` he phase Shift. thevv progress., et the. signal tlc Qu hf. the. ch Th ci; additional circu iur-.ther enhances. e selectiyitrflcecause oi the progressiyephase sh it. alreadyi notedi. if three, rather-than.- twoi. selectiye circuits ceceni..-

lllcred, the. phaser of; thet sishali at. the, third veircuit if; the. impressed. frequencyJ deu-lates from the: desir-ed irecueney i by, more.. than one ,-.tentlr of che; per.: cent, is twenty-two, degreesf,. with. correspondingly greater attenuation. and,A four circuits are; employed, this phase shift. would be. thirty-threedegrees. In the` latter case, the atteimatio-1gi.F of; thei undesired sig-nal`lr woul'd reduce its peak. amplitude to not. more than. four per een@ of the. response to: the; desiredi signal.. Selectiyityof; this, degree4 isf a thing:v quito. unattainable from passive networks off inductancearidi capacity to date. rllglfris. performance; is7 the V1fir1ore--lre,rnarkabl4e=,when itisrecol'lectedf., thatbe'- Cit-.usellhese.. circuitsL accept symmetrically-'fdisposed.` si-gi'lals.V Without'. additional-v attenuation beyond thatA inherent. in the'. I1 and Gi alone,

.there .,i s- ,r;1o corresponding-loss ofintelligence'from the side?. band'. energy associated Withthe in;- corning. signal.

W ltuen.I the. freeoscillation frcopi-lerici?l of the circuits 4t2', als is 2.. timesthefreuuency ci actuation.. of; theipeidodic shunts. 414.. 42.0 (71:1, 2.,. 3.. -the..re.spcpse of' the system to.. interference. represented. bri a step. iuhetionis.. essentiallyas describedv earlier;

. It". is. hotihtended to indica-te that.. thev switching frequencyv must be identical. with.. or; twice.: the received; frequency. as. it. is; apparent: that the b,erielits.y from.. the iriveioticn-u willv also accrue. though to a lesser deereeiiiftheishuntingaaction is. obtained at everifsecondvoltagei. zero or; every third voltage zero, etc.

Returning now to.- a consideration of; the

tra.I isileht,y the reaction; may` beisiimmariaedi Sim-- It, is. well'yknown that; 'the pist-:breathing thatthe circuttpossesses thermop- Thus, it mayv be said. that the arrangementof Figure, 13. which the successive circuits.. areL isochronous; with .eac1fi other ata frequencyv times.. the,v frequency of periodic shunt` actuation, a.. VeryI high degree. of; attenuation of. signals asymmetrically disposed. about, the` desiredsignal itetlpehcyin the. spectrum is.. obtained. while signais.. having components symmetrically disposed abcutthe. acceptance frequency of, the. system.. in the manner. which. would be expected in. an amplitude modulated. double. sideband' wave, are Rassedf, Without: attenuation exceeding or dii'ering;- ffrom. successive exposure. ci such a signal to-y aV series; oi selective. circuits. Some. improve.- riot-mt:v imsighal-to-noise ratio. transients. may be expected, though this probably lies in the range 3..-.6- clec-zilclels.l

To, obtain. a. further improvement. in. signal-to noise ratio, overI the; arrangement of. Figline 113', While retaining the advantageousselectivty char.- acteristic of the, said circuit. the. modification illustrated. in Figure'. Lei. is. indicated.v Fundamentallythe.departurcin thecircuit. of' Figure 14 lies.` in. the. fact that. theV cascaded; resonant. cir.- cuits do not. haine., isochronous. free.. oscillation ltreduencies... This,Y arrangement reduires... that thephaseot the periodic, shunts connected' across the. oscillatory circuits. be independently adjust.- able.. In thereceiyer. of' Figure, l'fi,V` therefore, the antenna. 5.011, is connected with. the. input. oi the tuned.. radio.. frequency amplifiers. 5cl, 5.0.3., the amplier 5,02. feeding the` signal channel; mixer 5,04, Whose.. oiltlglut5 is connected, with. the. tuned circuit. A controllable shunt 5.08 is connected across the.. circuit. ills, and the. output. of this combination. is. impressed. on. the. ampliiier. valve 51.0 coupled. to-l another tuned.. circuit. 51.2. par.- alleled.; by. the shunt. 5.1.4.e The output.y of' these elements... is... in, turn. deliyered.. to. the. detector. 51,6 linked to, they modulation.. frequency amplifier 5 |18, feedingtheload' devicefl.

.Anioscillator Sill. supplies.. thev required;rv locally geheratedffreouepcy to; the. mixer. 504 its. output frequency dilering from; the., received signalLfre.-

cuency by: substantially.- the desired', intermediate ireqilency.. In addition. the. oscillator; feeds/a .mixer 5.11.1J excited from.. the, radio; frequency amplifier. 50.3 situated. in the control.. channel.. The desiredcbeatehergfy fromtheimixer. 50,5.L is: ampli.- 1.1.C3.2l-.I.1.dl subjected. tothe. highly selectiveactonof intermediate `frequency amplifier.. 50.1 andy then delyeredtothe phase Shifters 5.0.91. andzl which permit adiustment..V of the phase of; the. voltages controlling; the, peri@ dic. shunts withL respectvto .the

desiredsighal. Thesource.- of...co.ntrol.voltages; for

thefmultiuibpatorffl lf toi-sutablycontrol itsphase.

and/or frequency with respect to the 'desired signal wave. In this linkage between the comparator'5l5 and multivibrator 5l I, suitable time delay means may be interposed to minimize the eects of momentary loss of the control signal or random variations in its phase or amplitude. The signals from the coupling valve l9 control the periodic shunt BOB substantially in the fashion previously described.

The action of the periodic shunt 5N is controlled from the coupling valve 528 excited from multivibrator 522 through the pulse former 523. A portion of the output signal from the pulse former 523 is also impressed on the comparator 524, receiving signals as well through the phase shifter 52! from the control signal channel and developing a control impulse impressed, via controlunit 525 on the multivibrator 522.

The circuits 506 and SI2 are adjusted to have mutually diering free oscillation frequencies and one or both may have resonant frequencies differing from the frequency of the desired signal. The difference between their frequencies preferably does not exceed Q divided by their geometric mean frequency for best results. When the apparatus is in operation, phase Shifters 509, 52| are adjusted for most eiiicient transmission of signals at the desired frequency. The detuning from the desired signal frequency is maintained sufdciently low that the natural selective characteristic of the circuits 506, 5l! does not interfere undulywith the development of reasonably high gain and the selective characteristic arising from the action of the periodic shunting circuits.

Let us assume now, for example, that one of the circuits 506 is tuned one-tenth of one percent on the high side of the desired signal frequency and the other of the circuits 502 is tuned onetenth of one per cent on the low side of the desired signal frequency, and that each circuit has a Q of the order of 100. Detuning of this magnitude in the assumed circuit leaves the response to the desired signal substantially unaiected, and the single signal selectivity characteristic does not depart essentially from that illustrated at 404 in 'Figure l2. As before, the selectivity characteristic presented to a signal of symmetrical nature such as that provided by amplitude modulating a carrier and transmitting both sidebands, is not the selectivity characteristic 404 but rather the combined passive selectivity characteristic of the circuits 506, 5I2 cascaded, without benefit of the yperiodic shunting action. The application of a step function to this signal chain `produces the well-known shock wave at the free oscillation frequency of the circuit 506. That portion of the energy of the shock wave which may be resolved into a signal having its voltage peaks coinciding with the periods of activation of the shunt 508 is destroyed, and the remaining shock Waves are subjected to slightly greater attenuation than that normally incident to such a circuit by virtue of ythe phase slippage occurring between successive actuations of the periodic shunt. On arriving in the circuit 5l2, the shock wave sets up a signal which is, however, displaced in phase with respect tion.

, Many modiiications and variations adaptingv the invention to specic environments but not departing essentially from the spirit of the invention will be obvious to those skilled in the art, for the widespread use of these principles is expected to permit far more efcient utilization of the necessarily limited radiation spectrum available and to materially extend the operating range of present transmitting facilities.

I claim:

l. In combination, an electric circuit including inductance and capacitance and having a resonant period determined by said inductance and capacitance, and a resistive impedance varying with a period 17,/2 times said resonant period (where n is any integer) effectively connected in shunt with at least a portion of said capaci` tance.

2. In combination, an electric circuit including inductance and capacitance and having a. resonant period determined at least in part by said inductance and capacitance, a variable conductance connected in shunt with at least a portion of said capacitance, and a control circuit periodically varying said conductance between relatively high values and relatively low values with a period 11,/2 times said resonant period, where n is any integer.

3. In combination, an electric circuit including inductance and capacitance and having a resonant period determined at least in part by said inductance and capacitance, a controllable conductance connected in shunt with at least a portion o f said capacitance, and a control circuit periodically varying said conductance between relatively high values and relatively low values in a manner essentially independent of forced oscillations appearing in said circuit and with a period of substantially half said resonant period.

4. In combination, an electric circuit including inductance and capacitance and having a resonant period determined at least in part by said inductance and capacitance, a controllable conductance operatively shunted across at least a portion of said capacitance, and a control circuit periodically varying said conductance between relatively high values and relatively low .values with a period related in predetermined 4manner to .said resonant frequency and a high .value duration of less than one ninth said resonant period.

.5. In combination, an oscillatory electric circuit with less than critical damping, a network of .controllable energy absorbing properties opera-- -tively associated with said circuit, a control de'- vice influencing the energy absorbing properties of said network, and a signal source connected with'said control device supplying a signal enhancing said energy absorbing properties for time lintervals not substantially exceeding one ninth 'vice influencing the energy absorbing properties of said network, a signal source supplying stimuli jwith a repetition rate of the order of twice the natural frequency of oscillation of said electric circuit and a duration not substantially exceed- -ing one ninth'the natural period of said electric circuit, and a connection between said signal vsource and said control device.

7. In combination a first signal li 1 ne, an oscillatory electric circuit with less than critical damping adapted for excitation in response to signals appearing on said iii-st signal line, a

network of controllable energy absorbing properties operatively associated with said circuit, a control device influencing the energy absorbing properties of said network, a controllable signal source supplying to said control device stimuli having a time duration not substantially exceeding one ninth the natural period of said electric circuit, and control apparatus varying the repetition rate of said controllable signal source in accordance with signals appearing on said first signal line.

8. In combination, an oscillatory electric circuit with less than critical damping resonant at a predetermined frequency, a network of controllable energy absorbing properties operatively associated with said circuit, a control device influencing the energy absorbing properties of said network with a periodicity different from that at which successive high absorption intervals would be caused to fall on a wave of said predetermined frequency at points separated by mr ("n is an integer), said difference not exceeding the Q of said electric circuit divided into 0.5 when expressed proportionately.

9. The method of enhancing the frequency discriminating properties of a frequency selective network which comprises absorbing energy from a region of said network at times when, in the presence of the desired signal, there would be no stored energy.

10. In electrical communicating apparatus, a signal line, first and second oscillatory electric circuits with less than critical damping adapted for excitation in response to signals appearing on said signal line, a first periodically active energy absorbing circuit associated with said first electric circuit, a second periodically active energy absorbing circuit associated with said second electric circuit, and phase adjusting apparatus relatively adjusting the phases of the operating stimuli and signals to cause the energy absorption from said electric circuits to occur at times displaced by a substantially integral number of right angles on the input wave supplied to said electric circuits.

11. In combination, a signal line, an oscillatory electric circuit with less than critical damping, a network having periodically varying energy absorbing properties associated with said electric circuit, and a transmission network characterized by a more sharply selective frequency-transmission characteristic than said oscillatory circuit connected in controlling relation between said signal line and said energy absorbing network.

12. In electric wave responsive apparatus, a first oscillatory electric circuit with less than critical damping characterized by a predetermined resonant frequency and Q, a second oscillatory electric circuit with less than critical damping characterized by a predetermined resonant frequency and Q, the ratio of resonant frequency to Q of said first electric circuit being greater than the ratio of resonant frequency to Q of said second electric circuit, and an energy absorbing network associated with said second electric circuit absorbing energy therefrom for time intervals not substantially exceeding one ninth the period of said second resonant frequency and a repetition rate related in predetermined manner to said second resonant frequency.

13. In combination, a first electric valve having input and output electrodes interconnected to reduce the apparent impedance of said valve during the operation of said valve and characterized by the passage of an electric current during the operation of said first valve, a second electric valve adapted to draw an electric current comparable with that passing incidental to the operation of said rst valve operatively connected in parallel with said first valve, and a control circuit alternatively disabling said valves.

14. In combination, an electrode structure adapted to project a beam of charged particles, a control electrode adapted to establish and interrupt said beam, a pair of control deiiecting electrodes disposed on either side of the normal course of said beam, a pair of target electrodes situated beyond said delecting electrodes in the direction of beam travel, a connection between one of said deflecting electrodes and one of said target electrodes, and a connection between the other of said deflecting electrodes and the other of said target electrodes, the poling of said connections being such that the apparent impedance between said deflecting electrodes is reduced in the presence of said beam.

15. In combination, first and second signal lines adapted for symmetrical excitation, a first set of unilateral conductors connected in opposition with a first polarity between said signal lines, a second set of unilateral conductors connected in opposition with reverse polarity between said signal lines, and an exciting circuit simultaneously applying opposite polarities to the junction points within said sets.

16. In signal responsive apparatus, a rst oscillatory electric circuit having less than critical damping, a second oscillatory electric circuit having less than critical damping adapted for excitation from said first circuit said circuits being tuned to substantially the same frequency, a first controllable energy absorber associated with said rst circuit, a second controllable energy absorber associated with said second circuit, and control apparatus periodically varying the energy Aabsorbing properties of said energy absorbers, said periodic variation occurring at a frequency substantially 2/11. times the tuned frequency of said rst and second circuits, where n is any integer.

17. In signal responsive apparatus, a rst oscillatory electric circuit having less than critical damping and a first resonant frequency, a second oscillatory electric circuit having less than critical damping and a second resonant frequency different from said first resonant frequency, a first controllable energy absorber associated with said first circuit, a second controllable energy absorber associated with said second circuit, and control apparatus periodically varying the energy absorbing properties of said energy absorbers.

GEORGE V. ELTGROTH.

REFERENCES CITED The following references are of recordin the file of this patent:

UNITED STATES PATENTS Number Name Date 1,794,878 Weagant Mar. 3, 1931 2,269,140 Bruck Jan. 6, 1942 2,393,785 Leeds Jan. 29, 1946 2,428,126 Nicholson Sept. 30, 1947 2,448,558 Stodola Sept. 7, 1948 2,477,963 Chapin Aug. 2, 1949 

